Machines such as excavators, loaders, dozers, motor graders, and other types of heavy equipment use one or more actuators supplied with hydraulic fluid from a pump on the machine to accomplish a variety of tasks. These actuators are typically velocity controlled based on an actuation position of an operator interface device. For example, an operator interface device such as a joystick, a pedal, or another suitable device may be movable to generate a signal indicative of a desired velocity of an associated hydraulic actuator. When an operator moves the interface device, the operator expects the hydraulic actuator to move at an associated predetermined velocity.
In some situations, it may be possible for a pressure of the fluid supplied to the actuator(s) to exceed a desired level. This over-pressure situation can occur, for example, when work tool movement becomes stalled (e.g., when the work tool strikes against an immovable object). In these situations, the actuator or other components of the associated system can malfunction or be damaged. Accordingly, care should be taken to avoid such occurrences.
Conventionally, over-pressure situations are dealt with in one of two different ways. First, a main pressure relief valve associated with the system can open when system pressure exceeds a desired pressure. High-pressure fluid from the system is then dumped through the open valve into a low-pressure tank, thereby reducing the pressure of the system. Although effective, this strategy can be inefficient, as the dumped fluid contains significant energy that is wasted. At the same time, the wasted energy is dissipated in the form of heat, which creates a cooling issue itself. The second way to deal with high-pressure is to implement a pump control strategy known as high-pressure cutout, which automatically reduces pump output upon detection of an over-pressure situation. The reduction in pump output allows for a corresponding reduction in system pressure as the pressurized fluid within the system is consumed. Although also effective, high-pressure cutout can cause a sudden and unexpected drop in power. In addition, high-pressure cutout, by itself, may not be responsive enough to ensure that harmful over-pressure spikes do not occur.
In some situations, a main relief valve may be used together with a high-pressure cutout strategy. Specifically, the pump can be controlled to reduce power as system pressures increase and, when the system pressures further increase and exceed a desired level, a main relief valve can open to protect system components from damaging extremes. This strategy, however, may still cause a drop in power that is unexpected and undesired by the operator.
The disclosed hydraulic control system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.